This invention relates to internal combustion engine ignition system testing and more particularly to an improved high voltage power supply for a spark plug test fixture.
Service facilities for internal combustion engines such as those used in automobile, aircraft and the like, generally have test fixtures for testing the operation of spark plugs. Such fixtures test spark plugs by applying a high voltage across the spark gap in the plug while the gap is subjected to high pressure. The high pressure is applied from a source of compressed air such as the standard air compressor found in most service facilities while the high voltage is applied from a power supply located within the test fixture. The "quench pressure" of a spark plug under test is measured by increasing the air pressure at the spark gap until the plug ceases to fire. If such spark plug is not capable of sparking or firing while subjected to a predetermined air pressure and a predetermined high voltage, the plug is discarded.
Various types of power supplies have commonly been used in the past for generating high voltages in spark plug test fixtures. One commonly used power supply involves the use of a vibrator and an ignition coil. A DC power source, such as a battery or rectified alternating current is applied to the vibrator which in turn drives the primary winding of the ignition coil. However, the vibrator causes the ignition coil to have a fluctuating peak output voltage which causes a very broad indication of the quench pressure for the spark plug. In addition to obtaining only a broad indication of the quench pressure for the spark plug, the vibrating contacts in the vibrator also produce a large amount of electromagnetic interference. In a second type of high voltage power supply, a DC power source is connected to charge a capacitor. When the charge on the capacitor exceeds the breakdown voltage of a breakdown device such as a neon filled discharge tube, the capacitor is discharged through the device to the primary winding of an ignition coil. The resulting high secondary voltage is applied to the spark plug under test. Both types of power supplies provide only a general indication of the quench pressure for a spark plug under test. One source of difficulty is in the wide variations or fluctuations in the peak output voltage applied to the spark plug during test. Still another difficulty with prior art high voltage power supplies for spark plug test fixtures is the inability or difficulty to adjust the peak output voltage. Since different types of spark plugs, such as aircraft and automotive spark plugs, are tested at different voltages, different power supplies have normally been necessary for testing different types of spark plugs.